Acoustical transducer calibrating system and apparatus

ABSTRACT

An acoustical transducer calibrating system including a differential pressure actuating device having an inner chamber for applying differential pressures to the transducer under test, and an outer chamber for vacuum sealing the device to the surface under test. Mounted within the inner chamber is an electrostatic actuator for exciting the transducer at selected frequencies so that its sensitivity can be determined for different operating ambient pressures.

United States Patent 1191 Lewis et al.

[ 1 July 1a, 1973 ACOUSTICAL TRANSDUCER [54] 2,558,550 6/1951 Fiske179/1751 A CALIBRATING SYSTEM AND APPARATUS [75] Inventors: :23??? E gagConn PrimaryExaminer--S. Clement Swisher AttorneyDarrell G. Brekke, JohnR. Manning et al. [73] Assignee: The United States of America asrepresented by the Administrator of the National Aeronautics and SpaceAdministration, Washington, D.C. [57] ABSTRACT [22] Filed: Oct. 14, 1971An acoustical transducer calibrating system including [21] Appl' 189290a differential pressure actuating device having an inner chamber forapplying differential pressures to the [52] US. Cl. 73/1 DV, 340/5 C,179/ l75.l A transducer under test, and an outer chamber for vac- [51]Int. Cl 601v 13/00 uum s ling the device to the surface under test. [58]Field of Search 73/ 1 DV, 4 R; Mounted within the inner chamber is anelectrostatic 340/5 C; 179/175.1 A actuator for exciting the transducerat selected frequencies so that its sensitivity can be determined for[56] References Cited different operating ambient pressures.

UNlTED STATES PATENTS 3,186,212 6/ l965 v Sharko 73/4 R 7 Claims, 3Drawing Figures I 40 i 38 26 20 I0 II I I r 21 I V k 2 24 24 36 3 44 42l" 22 46 .1 .4 I2 33 1 3/ PATENIEL Jill n 3. 744.294

ABSOLUTE PRESSURE GAUGE E2 vim? Fig.3

70 D. c. 50 POWER 7 SUPPLY 2 VAR. FREQ. OSCILLATOR 74 w RECORDERINVENTORS THOMAS L. LEWIS BY ROBERT B. COHN ATTORNEY ACOUSTICALTRANSDUCER CALIBRATING SYSTEM AND APPARATUS The invention describedherein was made by employees of the United States Government and may bemanufactured and used by or for the Government for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION '1. Field of the Invention The presentinvention relates generally to apparatus for calibrating acousticaltransducers and more particularly to an actuating device and associatedcalibrating system for determining the sensitivity of an installedacoustical transducer without requiring that it be removed from itsmount.

2. Discussion of the Prior Art In evaluating the aerodynamiccharacteristics of an aerodynamic surface, acoustical transducers aretypically utilized to measure the effects of air flowing across thesurface as evidenced by changes in surface pressure. The pressureresponsive diaphram of the transducers are usually mounted flush withthe surface. Incorporated in some types of transducers is a smallventing passage which allows the transducer to measure only thefluctuating portion of the surface pressure, since with venting there isno difference in static pressure across the diaphram, i.e., there shouldbe no difference in static pressure between the opposite surfaces of thediaphram. Because of the arnall size of these passages, there is arather high likelihood that they will become blocked and thus affect themeasurement provided by the transducer. Heretofore, in order todetermine whether the passages were open so that proper ventingoccurred, the test surface was usually placed in a large laboratoryvacuum chamber and small electrostatic devices were placed over thepassages to provide calibrating excitation of the transducer. Theventing check was made by reducing the pressure in the chamber, excitingthe transducer with the electrostatic device and then noting itsresponse as the chamber was allowed to return to atmospheric pressure.

In order to test the differential pressure sensitivity of thetransducer, it was necessary to remove it from its mount. This wasobviously a great disadvantage in that the removal was time consuming,and the removal and reinstallation of the transducer often caused damageto either the transducer or the mounting surface thereby making itdifficult to obtain the required flushness and proper seal between thetransducer and its mount.

SUMMARY OF THE PRESENT INVENTION It is therefore an object of thepresent invention to provide means for calibrating acousticaltransducers mounted in situ to an airfoil surface, or the like, andincluding a simple differential pressure actuating device that can bepositioned on the test surface over the transducer so that a ventingtest as well as calibration of the transducer can be accomplishedwithout either damaging the test surface or requiring removal of thetransucer.

Briefly, the present invention includes a differential pressureactuating device for use with an appropriate enrgizing and detectionsystem. The actuating device includes an inner housing providing aninner chamber for applying differential pressures at other than ambientstatic pressures, to the transducer under test, and

an outer housing which cooperates with the inner housing to provide anouter chamber for vacuum sealing the device to the surface under test.Mounted within the inner chamber is an electrostatic actuator which isused to excite the transducer at selected frequencies so that thesensitivity of the transducer can be determined at various ambientstatic surface pressures.

Among the advantages of the present invention are that a transducer canbe tested in situ and the time required to make the test is thus muchshorter than was heretofore possible.

Other advantages of the present invention will no doubt become apparentto those skilled in the art after having read the following detaileddescription of a preferred embodiment which is illustrated in theseveral figures of the drawings.

IN THE DRAWINGS FIG. 1 is a perspective view of a differential pressureactuating device in accordance with the present invention.

FIG. 2 is a cross section taken through a test surface and an actuatingdevice of the type shown in FIG. 1.

FIG. 3 is a block diagram illustrating a calibration test system inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to thedrawing, a preferred embodiment of a differential pressure actuatingdevice 10 in accordance with the present invention is shown inperspective view in FIG. 1. In FIG. 2, a cross section of the device 10is shown mounted to the surface 12 of a structure 13 which might forexample be an aircraft wing or fuselage. Device 10 is positioned onsurface 12 over the pressure sensitive end (diaphram) 14 of anacoustical transducer. Transducer 16 is hermetically sealed to structure13 at 18. A venting passage 15 is provided in structure 13 in registrywith a venting hole 17 in transducer 16 to permit static pressureequalizations of both the interior and exterior surfaces of the pressuresensitive diaphram of transducer 16.

Actuator device 10 includes a cylindrical cup-shaped outer housing 20having a cylindrical inner housing 22 concentrically disposedtherewithin and sealingly affixed to its inner wall 24 by the bolts 26.The inner wall of housing 22 and the centermost portion of wall 24 forma cylindrical calibration chamber 28 which is open at the lower end. AnO-ring seal 30 is 'mounted within an annular groove 31 at the lower endof housing 22 for sealingly engaging the test surface 12.

Mounted within chamber 28 is an electrostatic actua tor 32 such as theModel UA0023 manufactured by Brel & Kjaer. Actuator 32 includes aninsulative annular sleeve 33 which electrically isolates theelectrostatic electrode 34 from the inside wall of inner housing 22.Sleeve 33 is held firmly (with approximately 2 lbs. of force) againstthe surface 12 by a spring 42, but the electrostatic electrode 34 isspaced from the transducer diaphram by an air gap 35 of approximately0.005 inch. Electrical contact to actuator 32 is made through aconductor 36 which extends upwardly through an insulator 38-to providean external contact terminal 40.

The inner walls of outer housing 20 and the outer walls of inner housing22 cooperate to form an outer vacuum chamber 44 which is used to securethe device to surface 12. Outer housing 20 is provided with an annulargroove 46 for receiving an O-ring 48 for sealingly engaging surface 12.Exhaust tubes 50 and 52 form exhaust passages extending through housinginto chambers 44 and 28, respectively, so that the pressure therewithincan be reduced by an external vacuum pump. When the device is positionedon surface 12, as illustrated in FIG. 2, and the pressure within chamber44 is reduced, the imbalance in pressure between the top surface 11 andsurface 24 of outer housing 20 causes the device to be forced onto thesurface 12. This force along with the frictional resistance to lateralmotion provided by O-rings and 48 insures that device 10 is secured tosurface 12. O-ring 30 seals off inner chamber 28 so that the pressuretherewithin is completely independent of that in chamber 44. Actuatorelectrode 34 has openings 35 therethrough so that when the pressurewithin chamber 28 is reduced, the communication passages 14 cause thepressure at the diaphram 14 of transducer 16 to likewise be reduced(these holes are also an inherent part of the electrostatic actuator).

Alternating current and direct current signals are applied to terminalto cause an alternating electrostatic force to be developed across theair gap 35 separating electrode 34 and the transducer diaphram 14. Theelectrostatic force cause pressure variations at the AC signal frequencywhich are detected by transducer 16.

In FIG. 3 of the drawings, a calibration test system including theactuating device 10 is illustrated in block diagram form. The systemincludes a vacuum pump 60 which is coupled through a main cut-off valve62 and a pair of individual cut-off valves 64 and 66 to the exhausttubes and 52 respectively. An absolute pressure gauge 68 is alsoprovided between valves 64-66 and valve 62 for indicating the vacuumdrawn by pump 60. A DC power supply 70 and a variable frequencyoscillator 72 are electrically connected to terminal 40 for energizingthe electrostatic actuator 32. The output of transducer 16 is amplifiedby an amplifier 74 and the amplified signal is recorded by a recorder 76which may include a meter, chart or tape recorder.

In operation, calibrating device 10 is positioned on surface 12 asindicated in FIGS. 2 and 3, and valves 62 and 64 are opened while valve66 is closed. Pump is then energized to begin reducing the pressure inouter vacuum chamber 44 so as to secure device 10 in place on surface 12and at the same time form a seal for inner chamber 28. When the pressurewithin chamber 44 has been reduced to approximately 8 pounds per squareinch, as indicated by pressure gauge 68, valve 64 is closed and valve 66is opened so that inner vacuum chamber 28 can be exhausted. After thepressure in chamber 28 has been reduced to a selected value,electrostatic actuator 32 is energized with a one volt potential fromthe DC power supply and a 1 volt AC potential (at a selected frequency)from the variable frequency oscillator 72.

The frequency of the output signal developed by oscillator 72 is thenswept over a test range, such as between 20 hertz and 20,000 hertz, andthe amplified output of transducer 18 is recorded by recorder 76. Theamplitude of the recorded signal provides a clear indication of thesensitivity of transducer 18 at the selected pressure. The pressurewithin chamber 28 is then changed and the test repeated, and so forth.Since inner chamber 28 is completely independent from outer chamber 44,the pressure within inner chamber 28 can be changed to any selectedpressure so as to enable the sensitivity of transducer 18 to bedetermined at any number of pressure levels. When actuator 32 isenergized at a single selected frequency, the recorded output willindicate whether proper venting has occured, the sensitivity of thetransducer 16 at the selected frequency, and the pressure within innerchamber 28.

Although the actuating device of the present invention has beendisclosed in a form primarily suited for application to planar, or nearplanar, surfaces, it will be appreciated that the configuration ofhousings 20 and 22 can be appropriately modified to fit any of thevarious curved surfaces used to form airfoils. It will also beappreciated that the size of outer housing 20 can be increased and itsconfiguration modified so that a number of inner housings 22 can bepositioned therein for the purpose of simultaneously calibrating morethan one transducer at a time.

Whereas the present invention has been described in terms of a singlepreferred embodiment, it is contemplated that numerous modificationsthereof will be apparent to those skilled in the art. Accordingly, it isintended that the appended claims not be limited by the abovedisclosure, but be interpreted as covering all such alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for calibrating an acoustical transducer communicativelymounted to a surface, comprising:

an actuating device including means forming a first chamber having afirst opening on one side with sealing means disposed thereabout forsealingly engaging said test surface, means disposed within said firstchamber for developing acoustical forces for exciting said transducer,and means for securing said actuating device to said surface;

means for selectively changing the pressure within said first chamber;and

means responsive to the output of said transducer and operative toindicate its response to said acoustical forces.

2. Apparatus for calibrating an acoustical transducer as recited inclaim 1 and further including means for selectively changing thepressure within said first chamber.

3. An actuating device for calibrating an acoustical transducercommunicatively mounted to a surface, comprising:

means forming a first chamber;

means forming a second chamber disposed about said first chamber, saidfirst and second chambers having openings on a common side with sealingmeans disposed about the openings for sealingly engaging said surface;

means for reducing the pressure within said second chmaber whereby avacuum seal is provided about said first chamber for securing theactuating device to said surface;

acoustical force generating means disposed within said first chamber forexciting said transducer; and means for selectively changing thepressure within said first chamber.

4. An actuating device for use in calibrating an acoustical transducercommunicatively mounted to a surface, comprising:

first housing means forming an inner chamber having a first opening onone side with sealing means disposed thereabout for sealingly engagingsaid surface; second housing means disposed in cooperating relationshipwith said first housing means to form an outer chamber having a secondopening circumscribing said first opening, said second housing meanshaving sealing means disposed about said second opening for sealinglyengaging said surface;

acoustic force generating means disposed within said inner chamber forexciting said transducer; and

means for changing the pressure within said inner and outer chambers.

5. I An actuating device for use in calibrating an acoustical transduceras recited in claim 4 wherein said force generating means includes anelectrostatic actuator.

6. An actuating device for use in calibrating an acoustical transduceras recited in claim 4 wherein said means for changing allows thepressure within said inner chamber to be changed independent of thepressure within said outer chamber.

7. An actuating device for use in calibrating an acoustical transducer,comprising:

a cylindi'ical outer housing;

a cylindrical inner housing disposed concentric with said outer housingand cooperating therewith to form an inner chamber and an outer chamberboth of which have openings at one end;

sealing means disposed about each of said openings for sealinglyengaging a surface to which said transducer is communicatively mounted;

acoustic force generating means disposed within one of said chambers forexciting said transducer; and

means forming exhaust passages through which said inner and outerchambers may be evacuated.

1. Apparatus for calibrating an acoustical transducer communicativelymounted to a surface, comprising: an actuating device including meansforming a first chamber having a first opening on one side with sealingmeans disposed thereabout for sealingly engaging said test surface,means disposed within said first chamber for developing acousticalforces for exciting said transducer, and means for securing saidactuating device to said surface; means for selectively changing thepressure within said first chamber; and means responsive to the outputof said transducer and operative to indicate its response to saidacoustical forces.
 2. Apparatus for calibrating an acoustical transduceras recited in claim 1 and further including means for selectivelychanging the pressure within said first chamber.
 3. An actuating devicefor calibrating an acoustical transducer communicatively mounted to asurface, comprising: means forming a first chamber; means forming asecond chamber disposed about said first chamber, said first and secondchambers having openings on a common side with sealing means disposedabout the openings for sealingly engaging said surface; means forreducing the pressure within said second chmaber whereby a vacuum sealis provided about said first chamber for securing the actuating deviceto said surface; acoustical force generating means disposed within saidfirst chamber for exciting said transducer; and means for selectivelychanging the pressure within said first chamber.
 4. An actuating devicefor use in calibrating an acoustical transducer communicatively mountedto a surface, comprising: first housing means forming an inner chamberhaving a first opening on one side with sealing means disposedthereabout for sealingly engaging said surface; second housing meansdisposed in cooperating relationship with said first housing means toform an outer chamber having a second opening circumscribing said firstopening, said second housing means having sealing means disposed aboutsaid second opening for sealingly engaging said surface; acoustic forcegenerating means disposed within said inner chamber for exciting saidtransducer; and means for changing the pressure within said inner andouter chambers.
 5. An actuating device for use in calibrating anacoustical transducer as recited in claim 4 wherein said forcegenerating means includes an electrostatic actuator.
 6. An actuatingdevice for use in calibrating an acoustical transducer as recited inclaim 4 wherein said means for changing allows the pressure within saidinner chamber to be changed independent of the pressure within saidouter chamber.
 7. An actuating device for use in calibrating anacoustical transducer, comprising: a cylindrical outer housing; acylindrical inner housing disposed concentric with said outer housingand cooperating therewith to form an inner chamber and an outer chamberboth of which have openings at one end; sealing means disposed abouteach of said openings for sealingly engaging a surface to which saidtransducer is communicatively mounted; acoustic force generating meansdisposed within one of said chambers for exciting said transducer; andmeans forming exhaust passages through which said inner and outerchambers may be evacuated.